1. Field of the Invention
This invention relates to a method of drawing a thermoplastic resin film in the longitudinal direction.
2. Description of the Prior Art
In general, in a process of drawing a film in the longitudinal direction, i.e. the traveling direction of the film, the film was heated up to a drawing temperature. Then, the film was drawn by passing through drawing rolls having a different rotation speed, and cooled to solidify it by passing through a cooling roll.
Incidentally, a strain was generated in the film in the cross direction by the stress acting in the longitudinal direction during the drawing process, and the film was contracted in the cross direction (the phenomenon is called neck-in.). The film was moved on the drawing rolls, and abrasions were generated. Moreover, the degree of the neck-in varied according to the drawing conditions, resulting unevenness in the film thickness and in the film properties.
Heretofore, various means have been developed in order to solve the above problem. For example, by cleaning the drawing rolls, the holding force of the drawing rolls is raised, and dust which causes scratches on the film is removed. As another means, the drawing rolls are made of a material having a great friction coefficient. Another means comprises nipping the whole width of the film by nip rolls. Another means comprises nipping both sides of the film by nip rolls on the drawing rolls during drawing of the film (Japanese Patent KOKAI No. 60-104313). Another means comprises providing linear projections projected to upper and lower directions on both sides of the film and fitting the projections to grooves formed on pass rolls to avoid the movement of the film in the cross direction (U.S. Pat. No. 3,429,961).
While neck-in occurred in the cooling process using the cooling roll, neck-in also occurred in the drawing process when drawing a film in the longitudinal direction. The film moved in the cross direction by contraction through cooling, and by the tension in the longitudinal direction due to the drawing of the film, and abrasions occurred on the surface of the film. This should be a serious problem, particularly when the film is used as the base of a microfilm requiring severe qualities. Nevertheless, this problem has been overlooked, and no means has been taken for avoiding the neck-in in the cooling process.